1. Field of the Invention
The present invention relates to a process for controlling a hydraulic wash column (cf. FIG. 1, to which the numerical addresses which follow and the numerical addresses in the claims relate) which has a cylindrical shell (1) which delimits the column and within which one or more filter tubes (2) extend through the column parallel to the cylinder axis and have, in the vicinity of the second end of the column, at least one filter (3) in the filter tube wall which forms the sole direct connection between the filter tube interior under the pressure P1 and the interior of the column, in which                at least one stream of a suspension (4) which comprises crystals of a substance to be purified (to be removed in maximum purity) suspended in a mother liquor is fed continuously into the first end of the column (5) with a pressure P2 (for example by means of a pump (6)) which is greater than P1,        mother liquor (7) is conducted through the filters into the filter tube interior and out of the column via the filter tubes,        if appropriate, control liquor (9) is fed into the wash column at the first end of the column and/or between this end and the start of the filter,        the mother liquor and, if appropriate, control liquor flow in the column form a crystal bed (10) of the substance to be purified, said crystal bed having a buildup front (11) which faces the first end of the column and at which crystals of the introduced suspension add continuously to the crystal bed,        the crystal bed, by virtue of the force resulting from the hydraulic flow pressure drop of the mother liquor and, if appropriate, control liquor flow in the column, is transported (13) past the filters into the wash zone disposed between the filters and the second end of the wash column (12),        crystals are removed (14) continuously at the opposite end of the crystal bed to the buildup front,        the removed crystals are melted (15) and a portion of the melt is passed (16) through the crystal bed as a wash liquid stream coming from the second end of the column against the transport direction of the crystals and        the position of the buildup front is controlled with the aid of the flow rate of control liquor conducted into the wash column and/or with the aid of the flow rate of the suspension (4) conducted into the wash column.        
2. Description of the Background
In particular, the present invention relates to the control of a hydraulic wash column which is operated for the purifying removal of acrylic acid crystals from their suspension in contaminated acrylic acid, as is described in the documents WO 01/77056, WO 04/35514, WO 03/41833, WO 02/9839, WO 03/41832, DE                A 100 36 881, WO 02/55469 and WO 03/78378. The numerical addresses in this document always relate to the figures appended to this document.        
In this document, the terms cylindrical and tubular should be understood so as to comprise all geometric shapes (bodies) whose cross section is circular or circle-like (for example elliptical or polygonal (for example regular tetragon, hexagon or octagon).
In this document, the term mother liquor should be understood so as to comprise in particular melts composed of the substance to be purified and impurities and/or also solutions of the substance to be purified and solvents or solvent mixtures and also impurities. Equally, the term “melt of removed crystals as a wash liquid stream” should preferably comprise melts of removed crystals but also saturated solutions of removed crystals in solvents or solvent mixtures. Correspondingly, “melting of removed crystals” also comprises “saturating dissolution of removed crystals in solvents or solvent mixtures”. Acrylic acid, either itself or in the form of its salts or its esters, is of significance especially for the preparation of polymers for a wide variety of fields of use (for example adhesives, superabsorbants, binders).
The process according to the preamble of this document is known (cf., for example, EP-A 097 405, WO 03/041832, DE-A 100 36 881, WO 02/09839, WO 03/041833, WO 01/77056 and WO 03/063997).
The process generally follows a suspension crystallization which forms a very effective and inexpensive process for achieving a high purity of a desired chemical compound (of a chemical substance).
This is because, in the synthesis of a chemical compound, the desired substance typically does not occur as a pure product, but rather as a portion of a compound mixture which, in addition to the desired substance, comprises impurities such as unconverted starting compounds, solvents, by-products or undesired isomers.
When the desired substance is a crystallizable compound (for example acrylic acid) which is present in a liquid compound mixture after the synthesis process or can be converted into one (cf., for example, EP-A 1 015 411, DE-A 196 06 877, DE-A 103 36 386, EP-A 792 867, DE-A 102 35 847, WO 03/078378, WO 02/055469 in the case of acrylic acid or methacrylic acid), suspension crystallization (especially from the melt) is recommended as a purification process for the desired substance, which forms a very effective and inexpensive process for achieving a high purity of a desired chemical compound.
This utilizes the fact that impurities are substantially expelled from the crystal lattice in the course of growth of the crystals in a liquid and remain in the mother liquor. Suspension crystallization has the advantage over layer crystallization that it can be carried out in a continuous process. Moreover, the purity of the crystals is very high owing to their comparatively slow growth rate. In spite of the relatively slow growth rate, it is possible with suspension crystallization to achieve a high product throughput, since the crystallization is associated with a large total surface area available for growth owing to the large number of simultaneously growing crystals.
Even in a one-stage crystallization process, highly pure crystals of the desired compound are thus obtained. In principle, the suspension crystallization can be effected either from a solution or from a melt.
A decisive step which has a crucial influence on the purity of the crystallized target product is the removal of the highly pure crystals from their mother liquor, which comprises the impurities in enriched form and the uncrystallized fractions of the target product, by a solid/liquid separation process.
For the aforementioned separation task, it has been found to be useful to use wash columns. They comprise a generally cylindrical wall (shell) which delimits a process chamber.
Frequently disposed upstream of the process chamber is a distributor chamber into which the crystal suspension to be separated in the wash column is fed. On its path from the distributor chamber into the process chamber, the crystal suspension is distributed substantially uniformly over the cross section of the process chamber. In the process chamber, a denser crystal bed is obtained, especially by mother liquor removal, and conducted through the process chamber (in columns with forced transport of the crystal bed, this may either be from the top downward or from the bottom upward). The cross section of the process chamber is normally constant over its length. The wash liquid passed through the crystal bed in countercurrent is a melt of crystals which have been removed beforehand in the column (or a saturated solution thereof in solvents or solvent mixtures).
For the formation of a dense (compact) crystal bed in the wash column, different methods are employed in practice. In wash columns which operate under gravity, the crystal suspension is introduced into the column from the top and the crystal bed is formed in a sedimentation process with sole conveying action by gravity. Separation processes in such wash columns are not provided by the process according to the invention.
Wash columns with forced transport (or conveying) of the crystal bed differ from such gravimetric wash columns in that, in the conveying direction (or transport direction) of the crystal bed, at least one conveying force other than gravity acts.
In principle, wash columns with forced transport of the crystal bed are divided into pressure columns (also known as hydraulic wash columns or hydraulic columns) and into mechanical columns.
Mechanical wash columns comprise a mechanical forced conveying unit for the crystals. In the simplest case, this may be a semipermeable plunger which is permeable to the mother liquor but impermeable to the crystals in the suspension supplied (cf. FIG. 3 of WO 03/041832) and which shifts to generate the pressure for the compaction and conveying of the crystal bed. In a mechanical wash column, a crystal bed can also be compacted and conveyed by removal of the mother liquor through filters and mechanical transport of the crystals from the filter to the crystal bed by a rotating conveying element (for example screws, stirrers, helices or spirals) (cf. FIG. 4 of WO 03/041832). The filters may also be integrated into the rotating conveying elements. Separating processes in mechanical wash columns are not provided by the process according to the invention.
In the hydraulic wash columns relevant to the process according to the invention, the crystal suspension is conveyed at one end of the wash column into the cylindrical wash column under pressure (for example by pumps (6) according to FIG. 1 or by hydrostatic head). In parallel to the cylinder axis, at least one filter tube ((2) in FIG. 1) extends through the column and has, pointing away from the crystal suspension feed, at least one filter ((3) in FIG. 1) which forms the sole direct connection between the filter tube interior under a pressure P1 and the interior of the column.
Typically, the filter tubes project into the wash zone, but are no longer hollow in this region of the wash column (cf., for example, WO 01/77056, WO 03/41833 and WO 03/41832). This part of the filter tubes is also referred to as the filter tube displacer.
The liquid flow imposed by the feed column pressure P2 >P1 generates compaction of the crystals to a crystal bed and the conveying thereof. The mother liquor flows through the filters out of the wash column (beyond the filters, the pressure may be standard pressure, reduced pressure or superatmospheric pressure). The crystal bed has a so-called buildup front ((11) in FIG. 1) at which crystals of the introduced crystal suspension add on continuously. The buildup front thus denotes the transition from the suspension to the crystal bed and is indicated by a relatively abrupt rise in the crystal content in the suspension. It separates the suspension zone ((17) in FIG. 1) from the crystal bed. In many cases, the buildup front is also referred to as the filtration front. At the opposite end of the crystal bed to the buildup front, washed crystals are continuously removed ((14) in FIG. 1). This may be effected, for example, by means of a kind of rotor blade or with the aid of scrapers, which remove crystals continuously from the crystal bed. The continuous crystal removal may also be effected as described in WO 03/063997. There, suitable impulse on the wash melt brings about continuous disintegration of crystals from the crystal bed. The continuous addition of crystals to the buildup front on the one hand and the continuous removal of washed crystals at the opposite end of the crystal bed to the buildup front on the other hand define the transport direction of the crystal bed (it may either point from the top downward or from the bottom upward). The crystals removed from the crystal bed, preferably after they have been resuspended in pure melt (or in solvents or solvent mixtures) are melted by heat transfer (or dissolved, preferably in a saturating manner). A portion of the melt (of the pure melt) is removed as pure product stream ((20) in FIG. 1) and another portion of the pure melt is recycled (forced back) into the process chamber as wash liquid against the transport direction of the crystal bed at its end facing away from the buildup front. Typically, the wash liquid is at the melting point. Preference is given to effecting the resuspension in a separate chamber (the suspension chamber) adjoining the end of the crystal bed (typically not included in the wash column), into which, for example, the rotor blade introduces the removed crystals ((21) in FIG. 1). This suspension is then appropriately conducted in a melt circuit ((22) in FIG. 1) through a heat carrier ((15) in FIG. 1), by means of which the heat required to melt the crystals is introduced by an indirect route. Frequently from 70 to 80% by weight, in favorable cases (for example in the event of marked recrystallization in the wash zone) even from >80 to 100% by weight, of the molten crystals are removed as pure product ((20) in FIG. 1) from the melt circuit. The amount of pure product withdrawn is appropriately adjusted by means of a product regulating valve ((23) in FIG. 1). The conveying in the melt circuit is effected advantageously with a conveying pump ((24) in FIG. 1). The circulation rate in the melt circuit is advantageously from 2 to 30 m3/h, usually from 5 to 20 m3/h per m3 of removed purified crystals (calculated in molten form). In other words, the resuspension has a low crystal content favorable in performance terms, which promotes the conveying thereof.
How much pure melt penetrates from the melt circuit as wash melt into the wash column is appropriately controlled via the pressure in the melt circuit (suspension chamber) (this is determined indirectly by means of the adjustment of the product regulation valve). In general, this is a portion of the removed amount of crystals.
However, the melting of the removed crystals can also be undertaken directly within the wash column (for example by means of appropriately installed apparatus for heating at the end of the process chamber facing away from the buildup front). In this case, only a portion of the melt generated is withdrawn from the column. The other portion ascends as wash melt within the hydraulic wash column.
As a result of the conveying of the pure melt counter to the conveying direction of the crystal bed, the crystal bed, which has been impregnated with the mother liquor in particular, is virtually forced into the pure melt (and vice versa), and the mother liquor in the crystal bed is essentially simply forced back by the pure melt (“to form a substantially stable phase boundary between pure melt and mother liquor”).
In the steady state (operation), the result of this process at a defined height of the crystal bed (disposed between filter and crystal removal) is a wash front (“phase boundary”) which is defined as that point in the crystal bed where the highest temperature and concentration gradients occur (within the wash front, the phase transition is effectively from pure melt to mother liquor (or mixture of mother liquor and control liquor); in the wash front, the temperature virtually jumps from the lower mother liquor temperature to the higher pure melt temperature; above and below the wash front, the temperatures are substantially constant). Since pure melt and mother liquor (as already stated), expressed in coarse terms, collide with one another in the wash front, there is also a concentration jump of the undesired impurities at the height of the wash front from mother liquor concentration to pure melt concentration. The region from the wash front to the buildup front is referred to as the mother liquor zone and the region from the wash front to the end of the crystal bed facing away from the wash front is referred to as pure melt zone.
Since the crystallization temperature in the contaminated suspension is below the pure product melting point, there is also a temperature equalization of the cold crystals with the wash melt in the region of the wash front, in which the wash melt recrystallizes partly or fully. This recovers at least a portion of the wash melt. The other portion leaves the wash column together with the removed mother liquor through the filters and can, for example, be recycled into the recovery of the liquid compound mixture to be purified, or further purified ((25) in FIG. 1) and/or, if appropriate, used at least partly as control liquor which is yet to be described below ((9) in FIG. 1).
The further the crystallization temperature in the mother liquor is below the melt temperature of the pure product (or the saturation temperature of the wash solution), the more marked the described recrystallization of the wash melt (typical temperature differences are from 5 to 15 K). When recrystallization is quantitative, 100% of the molten crystals can be removed as pure product from the already described melt circuit ((22) in FIG. 1).
The cross section of the process chamber of a hydraulic wash column may be circular, oval or angular (for example regular-polygonal).
To achieve an adequate purifying action in the wash zone (the portion of the crystal bed which starts at the crystal removal and extends up to the start of the filter), the wash front has to be positioned at a certain minimum height above the crystal removal. In a similar manner, the buildup front also has to be well positioned in order to ensure effective operation of a hydraulic wash column.
Since the crystal bed is transported by the force resulting from the hydraulic flow pressure drop of the mother liquor (or mother liquor and control liquor) past the filters into the wash zone beyond the filters, it is energetically unfavorable for the operation of a hydraulic wash column when the concentration zone (the region of the crystal bed from the buildup front up to the start of the wash zone) is too elongated. With increasing length of the concentration zone, friction and the associated flow pressure drop increase. Conversely, an insufficient length of the concentration zone is disadvantageous in that it may be insufficient to form an adequately compacted crystal bed. Moreover, an excessively low flow pressure drop is incapable of fully satisfactorily transporting the crystal bed.
To ensure stable operation of a hydraulic wash column, i.e. to ensure a defined space-time yield at constantly good purifying action, constant compensation of external disrupting factors which impact upon the position of wash front and buildup front is required. Such disrupting factors may, for example, be variations in the flow rate of the suspension, changes in the crystal content in the suspension, variations in the crystal size distribution or else variations in the product mixture from the synthesis process fed to the crystallizer.
The position of the wash front is typically controlled by the setting of the amount of wash melt. This can be done, for example, as described in the documents DE-A 100 36 881 and WO 02/09839.
Since the position of the buildup front and filtration front is influenced by the hydraulic conditions (they determine the advance rate of the crystal bed) in the wash column, one possibility according to the teachings of DE-A 100 36 881 and WO 02/09839 is to influence the hydraulic pressure drop and hence to influence the advancing force in the wash column by pumping back a portion of the mother liquor removed by means of the filters (and if appropriate wash melt and also if appropriate control liquor) as control liquor ((9) in FIG. 1) into the wash column at the opposite end to the crystal removal and/or between this end and the start of the filter (if appropriate, control liquor is pumped in simultaneously at several points). The flow rate of control liquor to be recycled is appropriately adjusted by means of a corresponding control stream pump ((8) in FIG. 1), for example by changing the rotational speed and/or an additional regulating valve. However, useful control liquors are in principle also melts and/or solutions (which are preferably saturated with the substance to be purified) which comprise the substance to be purified and are different from mother liquor or identical to mother liquor, which can be drawn from external sources (i.e. not drawn from the wash column itself) (for example from the crystallizer and/or other process stages in the course of the preparation of the substance to be purified). It will be appreciated that the control liquors used may also be mixtures of control liquor types addressed above and below. Instead of by means of a pump, the control liquor may also be forced into the wash column by hydrostatic head. It is of course also possible to combine both feed variants. For example, the control liquor used may also be mother liquor which is withdrawn directly from the crystallizer used to prepare the suspension stream ((4) in FIG. 1) or the suspension disposed therein. The control liquor preferably does not have a higher purity than the mother liquor in order to substantially prevent recrystallization in the concentration zone. In other words, the starting melt used for the suspension crystallization itself is also useful as control liquor. Further useful control liquors are liquids in which the substance to be purified is firstly sparingly soluble to entirely insoluble and which secondly have a solidification point which is below the temperature of the crystal suspension fed to the wash column. When, for example, in continuous operation, the crystal bed ascends in the hydraulic wash column according to FIG. 1 (the buildup front shifts upward), the flow rate of the control liquor (and with it the hydraulic flow pressure drop of the mother liquor and control stream liquor in the wash column) increases, and, in the event of a descending crystal bed, it is reduced. The change in the flow rate of the control liquor can be carried out according to a defined characteristic, for example as a linear change in amount flow rate with time. Alternatively or additionally, the flow rate of the suspension (4) itself can correspondingly be increased or reduced to control the position of the buildup front.
When control liquor is forced into the wash column between the opposite end of the wash column to the crystal removal and the start of the filter at several points (at several heights) (this normally has to be done with different pressures; appropriately, these can be established by means of valves and correspond to the pressure existing at the particular feed point in the wash column or are slightly above them) (at the same time, the control liquors forced into the wash column may also have different chemical compositions; they are preferably chemically identical), an increase or decrease in the (total) control amount stream can be performed in such a way that the contribution of each individual control liquor stream is increased or reduced. Advantageously, the aforementioned increase or decrease is effected in such a way that the ratio of the control stream volumes fed per unit time at the different points relative to one another remains constant. It will be appreciated that the liquor control streams may also comprise “blank streams”. These are control liquor streams whose magnitude is kept stable irrespective of the behavior of the buildup front. They essentially form a base contribution to the control. Preferably only one control stream will be used, and will preferably be forced (pumped) (with the pressure P2) into the wash column (into the suspension zone) at the opposite end to the crystal removal. Advantageously, this control stream consists exclusively of liquor conducted out of the wash column through the filter tubes ((9) in FIG. 1) and is appropriately conducted (pumped) into the wash column by means of the control pump S ((8) in FIG. 1).
In principle, however, the control liquor stream can also be fed into the wash column by feeding the control liquor stream, for example upstream and/or downstream of the pump (6), into the suspension stream (4) to be fed to the wash column.
To monitor the position of the buildup front and filtration front, both WO 02/09839 and DE-A 100 36 881 recommend optical detection of the position. However, a disadvantage of optical detection of the position is that, at least at the point of detection, the cylindrical shell of the wash column has to consist of a transparent material (for example glass). However, the shell of the process chamber of a hydraulic wash column is preferably (cf., for example, WO 03/041832) manufactured from metal. Although it is possible in principle to install transparent windows in a metal wall, pressure-tight (as is required in the case of a hydraulic wash column) manufacture thereof is not simple in engineering terms and their use, especially in the case of hazardous substances, is not uncontroversial in safety terms. Furthermore, point monitoring through windows with restricted view is not fully satisfactory. Instead, what is desirable is a monitoring variant which captures (views) the entire length of the concentration zone. In addition, optical sensors are comparatively expensive.
It was an object of the present invention to provide an improved process for regulating the position of the buildup front within a hydraulic wash column, in which the flow rate of control liquor and/or suspension to be conducted into the hydraulic wash column at the particular time is determined by using a more advantageous parameter than in the prior art.